1. Field of the Invention
The present invention relates to a fixing device for electrophotographic recording apparatus, such as electrophotographic printers, copiers and facsimiles.
2. Related Art
The fixing device used for electrophotographic recording apparatus conventionally has a heating roller as a first roller and a pressure roller as a second roller, and causes recording paper with unfixed toner adhering to it to pass through the pressed contact portions of the rollers to thereby press and fuse toner to the recording paper.
The heating roller includes a heating source inside the mandrel (a halogen lamp for example), and the mandrel is covered with an elastic body having on its outer surface a parting agent layer formed, which is high in water and oil repellency (silicon rubber for example).
Because the elastic body is pressed with the pressure roller, a deformed portion is formed in the elastic body (hereafter the length of the deformed portion is referred to as a nip amount). Therefore, the toner is fixed to the recording paper as the paper having the unfixed toner adhering to it passes along the deformed portion of the elastic body.
To control the surface temperature of the heating roller, a temperature sensor (a thermistor for example) is so arranged as to contact the surface of the heating roller.
With conventional fixing device, there has been a problem that due to the low heat conductivity coefficient of the elastic body even when the heating source is made to generate heat, it takes time for the surface temperature of the heating roller to reach a specified temperature, so that the start-up of the device takes a long time.
On the other hand, to increase the printing speed, it is necessary only to increase the nip amount of the portion of the heating roller, against which the pressure roller is pressed, to provide a contact time sufficient to fuse and fix the unfixed toner to the recording paper. In other words, supposing the amount of deformation of the elastic body is constant, the outside diameter of the rollers needs to be increased to obtain a greater nip amount. However, as the outside diameter of the rollers is increased, the device becomes larger and the recording paper that has passed the contacted portions with pressure of the rollers is likely to wind itself around the roller. Thus, to increase a greater nip amount without increasing the roller outside diameter, it is necessary to only increase the thickness of the elastic body with low heat conductivity coefficient. Here arises a problem, however, that because heating the elastic body of a great thickness takes time, the temperature rise for startup of the device takes time.
The present invention has as its object to provide a fixing device, a method of temperature control of this device, and a method of manufacturing rollers of this device, which make it possible to increase the printing speed without increasing the roller outside diameter and which enable the surface temperature of the heating roller to reach a predetermined temperature in a short time.
To achieve the above object, according to the present invention, there is provided a fixing device for passing recording paper, which has unfixed toner adhering to it, through first and second rollers mutually pressed together, and fusing the toner under heat and pressure and fixing to the recording paper, wherein the first roller heated by a heating source comprises an elastic body provided around a mandrel and having a high heat resistance and a low heat conductivity coefficient and, a heat layer with a small heat capacity formed in tight contact with the circumferential surface of the elastic body, and a parting agent layer high in water and oil repellency formed in tight contact with the circumferential surface of the heat layer.
To achieve the same object, a method for temperature control of the fixing device according to the present invention for passing recording paper having unfixed toner adhering to it through a first and a second roller heated by heating sources, and fusing the toner under heat and pressure and fixing it to the recording paper, comprises the steps of bringing a first temperature sensor into contact with a circumferential surface of an end portion of the first roller not contacting the recording paper; bringing a plurality of second temperature sensors into contact with a circumferential surface of the center portion of the second roller contacting the recording paper and also to contact with the circumferential surface of the end portion of the second roller not contacting the recording paper; setting a predetermined temperature of the circumferential surface of the first roller to be maintained; determining a temperature difference between the circumferential surface of the center portion of and the circumferential surface of the end portion of the second roller from detected temperatures by the plurality of second temperature sensors after the recording paper has passed the first and second rollers; and obtaining a temperature of the circumferential surface at the center of the first roller from the determined temperature difference and a detected temperature by the first temperature sensor, wherein the center-portion circumferential surface temperature is lower than the predetermined temperature, electric power is supplied to the heating source to heat the first roller.